Those who know Ms. Stefanik well consider her smart and talented but hollow at the core. She is an individual of “ambition unmoored to principle,” as Barbara Comstock, a Virginia Republican who served in Congress with Ms. Stefanik, told me.

Margaret Hoover, the host of “Firing Line,” who worked in the Bush administration with Ms. Stefanik and was consulted by her before her run for Congress in 2014, described her as a person of “sheer ambition and not principled at all.” Another Republican — a former member of Congress who served with Ms. Stefanik and worked closely with her — also spoke to me of her towering ambition, invoking Icarus, the Greek mythological character. “She’s flying too close to the sun,” said this person, who requested anonymity in order to speak openly about her transformation…

But the most important inflection point was the first impeachment trial of Mr. Trump, in 2019. Ms. Stefanik accused Adam Schiff, the Democratic chairman of the committee overseeing the impeachment trial, of trying to silence Republicans and clashed with the former U.S. ambassador to Ukraine Marie L. Yovanovitch, whose testimony about Mr. Trump was damning…

“She became a celebrity,” I was told by a Bush administration colleague of Ms. Stefanik’s who requested anonymity so this individual could speak freely about what is still a very sensitive subject. Until that point, this person said, “she hadn’t gotten fully on board the Trump train. Then she was put into first class, and she couldn’t get off. And first class is pretty plush.”